The embodiments disclosed herein relate generally to wind turbines and more particularly to a system and method for protecting wind turbine generators from undesired torsional oscillations.
The quality of power distributed through modern electrical distribution systems continues to be an issue concerning operators of large systems. One such power quality problem is known as voltage flicker. Voltage flicker is a voltage dip that is of a magnitude sufficient to have an objectionable effect on other loads connected to the same circuit. The disturbance may be experienced as only blinking lights, but the magnitude and the frequency of the occurrences determine flicker's impact on system users.
FIG. 1 illustrates a common voltage flicker scenario. Flicker-producing loads 110 on system 100 are typically caused by large motors, welders, or arc-furnaces. These loads are characterized by high inrush currents of relatively short duration, as experienced in the starting of a motor. The motor's inrush current is typically of a low power factor, and causes a voltage dip of increasing magnitude along the feeder up to the point of the load's connection. This causes voltage flicker problems between the load and the source 120, which, when severe enough often leads to a user complaint 130.
The distribution series capacitor 140 has long been recognized as a cost-effective solution to these types of flicker problems. Unfortunately, distribution-class electrical power lines equipped with a distribution series capacitor are subject to two distinct and potentially damaging phenomena, ferroresonance involving transformers and self-excitation of motors during starting. Ferroresonance can be a severe and rapidly building oscillatory overvoltage condition caused by system non-linearities that can appear when power transformer cores saturate. These non-linearities interact with the series capacitor to produce a low-frequency resonant condition, often in response to large inrush currents following breaker operations. Self-excitation of induction motors is a potentially damaging condition that can occur on the same system. The term “self-excitation” refers to sub-harmonic oscillations that may occur in an electric supply circuit that includes series capacitors. The sub-harmonic oscillations result from the interaction between the series capacitors and an induction motor when the motor is in the process of starting. These oscillations are typically characterized by motor starting problems and sustained overcurrent conditions.
When ferroresonance occurs, immediate action should be taken to prevent damage to other equipment. Ferroresonance is a rapidly occurring, high magnitude, and low frequency oscillation capable of reaching power system voltage levels of 100-200% above normal for brief periods. When self-excitation occurs, low-frequency oscillations are produced as the motor starting sequence fails. The motor will search for the proper operating frequency, which will cause large current surges as the shaft acceleration alternates.
Power generation sites (e.g., thermal prime movers, induction generators, wind turbines, etc.) are often located very far from load centers. To enable the transmission of power over long distances, the use of series capacitors is often employed to raise the power limits of the resulting long transmission lines. The series capacitors can cause series-resonant oscillations, which have been known to cause damage to generator shafts. Damage could also be inflicted on wind turbine power transmission and control components.
The series-resonant oscillations occur at a sub-harmonic of the supply frequency (typically 60 Hz in North America). This effect has become known as subsynchronous resonance (SSR). The most famous incident involving SSR occurred in 1970 and again in 1971 at the Mohave Generating Station in southern Nevada, USA. A generator experienced a gradually growing vibration that eventually led to a fracture of the shaft section between the generator and the rotating exciter. Investigations determined that an electrical resonance at 30.5 Hz produced torque at 29.5 Hz (the 60 Hz compliment frequency), which was near coincident with the frequency of the second torsional vibration-mode of the turbine-generator at 30.1 Hz. This interaction between the series capacitors and the torsional system is an example of subsynchronous resonance.